Chapter 7:
The Theory of the Atom
The Present Day Model of
the Atom: The Quantum
Mechanical Model
SUBATOMIC PARTICLES
 Atoms are composed of three subatomic
particles:
1. PROTON
2. NEUTRON
3. ELECTRON
The Proton
 Positively Charged
 Located in nucleus
 Mass equal to the
mass of a neutron
 Discovered by
Goldstein
The Neutron
 No Charge
 Located in nucleus
 Mass equal to the
mass of a proton
 Discovered by
Chadwick
The Electron
 Negatively Charged
 Located outside the
nucleus
 Mass approximately
1/2000th the mass of
a proton.
 Discovered by
Thomson
Atomic Structure
 Atomic number: the # of
protons = the # of electrons in
an atom
12
 Atomic mass: the average
mass of all isotopes (not
rounded to a whole number)
 Mass number: the # of
protons & neutrons in an
atom (atomic mass rounded
to the whole number)
6
Calculating # of P, N & E
# Protons = atomic number = # Electrons
(Atoms are neutral therefore the # of
positive protons must equal the # of
negative electrons)
Mass Number = # protons + # neutrons
= # of nucleons
# Neutrons = mass number - atomic number
Atomic Symbols
Show the mass number and atomic
number
Give the symbol of the element
mass number
23 Na
atomic number 11
sodium-23
Atomic Symbols

27
13

56
26
Al
# P ____ # E ____ # N ____
Fe # P ____ # E ____ # N ____
Isotopes
 Atoms with the same number of protons,
but different numbers of neutrons.
 Atoms of the same element (same atomic
number) with different mass numbers
Isotopes of chlorine
35Cl
37Cl
17
17
chlorine - 35
chlorine - 37
Each one of these is called a nuclide of chlorine.
Isotopes
Average Atomic Mass
 Average atomic mass is an average of the
atomic masses of each isotope of that element.
Since there are different percentages of each
isotope in existence, the relative abundance is
also considered.
Atomic Mass = (%)(mass)+ (%) (mass) + (%)(mass)
100
Sample Problem
Neon has three naturally occurring isotopes. Neon-20 has
a mass of 19.9924 grams and an abundance of 90.92%,
Neon-21 has a mass of 20.9940 grams and an abundance
of 0.257%. Neon-22 has a mass of 21.9914 grams and an
abundance of 8.82%. Determine the average atomic mass
of neon.
(19.9924)(0.9092) + (20.9940)(0.00257) + (21.9914)(0.0882)
20.171 grams
20.2 grams
Development of the
Atomic Model
 Democritus – 400 BC
 Greek philosopher
 “Thought” about the idea of the world
composed of small particles called “atomos”
 The rest of the universe was empty space
Rejection of atomos
 Aristotle rejected Democritus’ ideas
claiming that empty space did not
exist.
 Aristotle claimed the elements
consisted of earth, fire, air and
water.
 As one of the most influential
philosophers of his time, he lead
people to reject the idea of atomos
from Democritus. (held science back
for 2000 years)
Development of the
Atomic Model
 J Dalton -- 1803
 “Atoms” Model
 Matter is composed
of indivisible particles
called atoms.
Dalton’s Atomic Theory
1.
2.
3.
4.
All matter is made of atoms.
Atoms are indivisible and indestructible.
All atoms of one element are identical.
Atoms of a specific element are
different from atoms of other elements.
5. Atoms combined in small whole number
ratios.
6. In a chemical change, atoms are
neither created nor destroyed, they are
just rearranged.
2 Parts of Dalton Incorrect
 #2 – atoms are composed of smaller
particles – electrons, protons and
neutrons
 #3 – isotopes exist – atoms of the same
element with different amounts of
neutrons – not identical
Crooke’s tube
 Found that there was a ray
traveling from the cathode to the
anode in the tube.
 The ray produced green flashes
on a zinc sulfide coating at the
other end of the tube.
 A cathode ray was a stream of
charged particles.
 TVs have cathode rays striking
light-producing chemicals coated
on the screen.
Go to animation of
a crooke’s tube
crooke's tube animation
Development of the
Atomic Model
 JJ Thomson - 1897
 “Plum Pudding” Model
 The atom is a positive
sphere with negative
electrons dispersed
throughout.
J.J Thomson
Thomson’s Cathode Ray
Tube Experiments:
 Found that cathode rays
were composed of
charged particles that
were attracted to the
positive pole of a magnet
 Concluded that the
particles must be negative
and called them electrons
Also: Demonstration of
cathode ray tube
cathode ray tube
Millikan’s Oil Drop Experiment
 Determined the charge
on an electron to a
great degree of
precision (within 1% of
the actual value).
 By varying the voltage
in this apparatus, he
could vary the rate at
which the oil drops
would fall.
Go to animation of
Millikan’s Oil Drop
Experiment
oil drop experiment
Development of the
Atomic Model
 E. Rutherford - 1910
 “Nuclear” Model
 The atom has a
positive center with
negative electrons
around it.
Go to learners tv.com
animation of Rutherford’s
experiment
 rutherford's gold foil experiment
Rutherford’s Gold Foil Experiment
First Conclusion
Since alpha particles are positive and
they are being deflected, they must be
hitting something positive.
NUCLEUS IS POSITIVE!!!!!
Rutherford’s Gold Foil Experiment
Second Conclusion
The object that was deflecting the alpha
particles must be very small because only 1
out of every 10,000 particles was deflected.
NUCLEUS IS SMALL!!!!!!
Rutherford’s Nuclear
Model
The atom had a “nucleus” that was small,
positive and dense.
Rejection of Rutherford’s
Model
The scientific community rejected the
nuclear model. Scientists questioned the
following:
“What prevents the negatively charged
electrons from falling into the positively
charged nucleus, resulting in the
collapse of the atom?”
Development of the
Atomic Model
 N. Bohr --- 1913
 “Planetary” Model
 Bohr proposed this
model to help solve
Rutherford’s problem.
 It looks like the planets
around the sun.
Bohr stated that:
 Electrons are constantly moving in a
fixed path around the nucleus.
 Since they are moving, they do not
collapse upon the nucleus.
The current model
 Quantum Mechanical Model
 The electron cloud is more
complex than “orbits” around
the sun.
 It is thought to have various
energy levels and sublevels,
each of which has a different
size, shape and orientation
around the nucleus.
Quantum Mechanical Model
 The current model is based on the probability of
finding an electron in a particular spot around the
nucleus.
 Complex mathematical calculations led to graphs
such as these:
Reactions of Elements
 In an ordinary chemical reaction, atoms are
rearranged to form new materials, but the
original identity of the atoms remains the
same. (no new element) The electron cloud is
involved.
2 Na + Cl2  2 NaCl
 In a nuclear reaction, atoms change into new
types of atoms, releasing radiation in the form
of particles or rays. (new element) The
nucleus is involved.
226
Ra 
222
Rn + alpha particle (α)
Unstable Nuclei
 Unstable nuclei emit radiation through a
process called radioactive decay.
 They will continue to decay until a stable
nucleus is formed.
 Stability is determined by the neutron to
proton ratio in the nucleus.
Alpha Radiation
 Alpha Particle: [ α ]
4
 ( 2 He) He Nuclei
+2 Charge
Mass = 6.64 x 10-24 grams
Can be stopped by Cardboard
4
 ALPHA DECAY – release of 2 He
238
234
4
U

Th
+
He
92
90
2
Notice that the sum of the mass numbers and the
atomic numbers is equal on each side.
Beta Radiation


Notice that the
sum of the
Beta Particle: [ β ]
mass numbers
0
and the atomic
( -1 e) Electron
numbers are
-1 Charge
equal on each
Mass = 9.11 x 10-28 grams
side.
Can be stopped by Aluminum
 Beta Decay -- Neutron => Proton + Electron
Overall loss of a neutron, but the proton goes to the nucleus of a new
element and an electron is radiated
14
6
C

14
7
N +
0
-1
e
Gamma Radiation
 Gamma Ray: [ γ ] High-Energy Photons
0 Charge
Mass = 0 grams
Can be stopped by thick blocks of Lead
 Gamma Decay – Only Energy is Given Off
60
60
Co

Energy (γ)
27
27 Co +
Notice that the mass numbers and the atomic
numbers are equal on each side.
RADIATION ANIMATION
alpha beta gamma rays
Strength of the Three
Types of Radiation
Patterns in Nuclear Stability
Like Charges REPEL, therefore lots of protons
together doesn’t make sense.
However, at close distances a “Strong Nuclear Force”
exists between protons and neutrons.
The “Neutron to Proton” ratios of Stable Nuclei
INCREASE with Increasing Atomic Number.
More protons require more neutrons to overcome the
repulsive forces among like charges.
Band of Stability Graph
Band of Stability
 A band of area within which all stable
nuclei are found.
 The Band of Stability ends with Bismuth.
 All nuclei w/ 84 or more protons are
radioactive.
Complete the equations:
 1)The alpha decay of iridium-174
 2) The beta decay of platinum-199
Half-life
 Half-life (t½)
 Time required for half the atoms of a
radioactive nuclide to decay.
 Shorter half-life = less stable.
RADIOACTIVE DECAY
 DECAY
Other Nuclear Reactions
 Fission – the splitting of atoms with the
release of energy
 Fusion – the combining of smaller atoms
to form larger atoms with the release of
energy
FISSION
the splitting of atoms with the
release of energy
1 g of 235U =
3 tons of coal
235
92
U
FISSION
 chain reaction - self-propagating reaction
 critical mass mass required
to sustain a
chain reaction
FISSION ANIMATION
 fission reaction
FUSION
 combining of two nuclei to form one
nucleus of larger mass
 thermonuclear reaction – requires temp
of 40,000,000 K to sustain
 1 g of fusion fuel =
20 tons of coal
 occurs naturally in
stars
2
1
H H
3
1
FUSION ANIMATION
 fusion reaction
FISSION VS FUSION
F
I
S
S
I
O
N
 235U is limited
 danger of meltdown
 toxic waste
 thermal pollution
F
U
S
I
O
N
 fuel is abundant
 no danger of
meltdown
 no toxic waste
 not yet sustainable
THE END